PURPOSE: To enable measurement with high accuracy without requiring cleaning and drying operations, and suspending dyeing, by measuring optical characteristics of a dyed thin film in a dyeing-stabilized transparent soln.

CONSTITUTION: A thin film 2 made of a high mol.wt. substance, such as natural protein is formed on one side of a transparent glass base 1, and the obtained body to be dyed is immersed in a dyeing soln. 3 made of an acidic dye to dye it in prescribed conditions. Then, it is immersed in an acidic soln. and its light transmittance is measured there with a light guide type photometer. A light emit ting guide 5 and a light receiving guide 6 are arranged at a prescribed interval, and each of their end faces 7, 8 is opposite to each other across the body. The light transmittance is determined by measuring an amt. of light received with the guide 6, and when additional dyeing is necessary, the body is again immersed in the dyeing soln. to dye it for a time length estimated in accordance with the measured result. When it is overdyed, it is immersed in an alkaline decoloring soln. to decolor it for a estimated time.